Energy storage technology is attracting more and more attention in recent years. As the applicability of energy storage technology is extending to mobile phones, camcorders, notebook PCs and even electric automobiles, there is a growing demand for high energy-density batteries as power sources for such electronic devices. Under these circumstances, lithium secondary batteries are considered as the most promising batteries and research on lithium secondary batteries is being actively undertaken.
Many secondary batteries are currently available. Of these, a typical example of the lithium secondary batteries developed in the early 1990's includes an anode made of a carbon material capable of intercalating/deintercalating lithium ions, a cathode made of a lithium-containing oxide and a non-aqueous electrolyte solution containing a lithium salt in a mixture of organic solvents.
Ethylene carbonate, propylene carbonate, dimethoxyethane, γ-butyrolactone, N,N-dimethylformamide, tetrahydrofuran, acetonitrile, etc. are currently widely used as organic solvents for non-aqueous electrolyte solutions. However, electrolyte solutions containing these organic solvents are prone to oxidation during long-term storage at high temperatures. As a result of such oxidation, gases are generated and deform the stable structure of batteries. In a serious case, heat is generated inside batteries when overcharged or overdischarged and causes internal short circuits of the batteries, posing the danger of fire or explosion of the batteries.
Numerous attempts to solve such problems have been made, for example, (1) by using porous polyolefin separators having a high enough melting point to prevent melting in high-temperature environments or (2) by adding flame retardant solvents or additives to electrolyte solutions to improve the safety of batteries at high temperatures.
However, a high melting point of a general polyolefin separator for a lithium secondary battery is accomplished by increasing the film thickness. This relatively decreases the amount of an anode and a cathode to be loaded, inevitably leading to a reduction in the capacity of the battery. Since the polyolefin film has a melting point around 150° C. because of the characteristics of its materials such as PE and PP, the separator may be melted by drastic internal heat of the battery arising from the oxidation of an electrolyte solution when overcharged. Such melting causes internal short circuits of the battery, and as a result, the problems of fire and explosion of the battery are difficult to avoid.
Japanese Unexamined Patent Publication No. 1997-259925 discloses the addition of a nonflammable gas having a boiling point not higher than 25° C. during assembly of an electrolyte solution. Several patent publications including Japanese Unexamined Patent Publication Nos. 2006-179458 and 2005-190873 describe the addition of phosphoric acid esters to carbonate electrolyte solutions for the purpose of ensuring nonflammability of the electrolyte solutions. U.S. Pat. No. 6,797,437 describes the addition of at least 30% of a nonflammable solvent such as a perfluoroalkyl or perfluoroester compound. However, injection of the nonflammable gas causes swelling of the battery and involves complex battery assembly steps. The phosphate additives cause deterioration of battery performance owing to high reduction potentials thereof. Furthermore, the perfluoroalkyl compound undergoes phase separation with an electrolyte solution containing an organic solvent. Such phase separation leads to the eduction of lithium salts.